Competition of optical transitions between direct and indirect bandgaps in Ge_1-xSn_x

Temperature-dependent photoluminescence (PL) study has been conducted in Ge_1-xSn_x films with Sn compositions of 0.9%, 3.2%, and 6.0% grown on Si. The competing between the direct and indirect bandgap transitions was clearly observed. The relative peak intensity of direct transition with respect to the indirect transition increases with an increase in temperature, indicating the direct transition dominates the PL at high temperature. Furthermore, as Sn composition increases, a progressive enhancement of direct transition was observed due to the reduction of direct-indirect valley separation, which experimentally confirms that the Ge _1-xSn_x could become the group IV-based direct bandgap material grown on Si by increasing the Sn content.